Field of the Invention
The present invention relates to: an excessive heating protection circuit for a fusing portion; and an image forming apparatus having the same.
Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As is well-known, a fusing portion is provided with a heat roller and a pressure roller that jointly form a nip area for fusing.
A print medium with unfused toner thereon is conveyed to the nip area for fusing. When the print medium passes through the nip area for fusing, the heat roller and the pressure roller apply heat and pressure, respectively, to the print medium. The toner is firmly fused onto the print medium accordingly.
In order not to excessively heating (overheating) the heat roller, the fusing portion is further provided with a temperature sensor that detects the temperature of the heat roller and outputs an analog voltage indicating a detected result. This output voltage is input to an excessive heating protection circuit that is mounted on a controller circuit substrate of an image forming apparatus. A reference voltage defining excessive heating of the heat roller is also input to this excessive heating protection circuit. Commonly, reference voltages are obtained by dividing supply voltage, i.e., constant voltage Vcc, with resistors. The excessive heating protection circuit compares the input temperature signal to the reference voltage with a comparator. With reference to a comparison result, the excessive heating protection circuit is allowed to protect the image forming apparatus from excessive heating (refer to Japanese Unexamined Patent Publication No. H09-022218 and No. 2014-077847, for example).
The fusing portion may be provided with a plurality of temperature sensors. These temperature sensors do not all necessarily have the same specifications. This means, these temperature sensors may be built to output voltage having different temperature characteristics, for example. In this case, suitable reference voltages for these temperature sensors need to be predetermined. Reference voltages are most commonly obtained by dividing voltage with resistors, as described above. So, the manufacturer of the image forming apparatus needs to prepare voltage-dividing resistors having a resistance value that is different depending on the excessive heating protection circuit. Furthermore, the manufacturer's image forming apparatuses may use different types of temperature sensors depending on the model. In this case, the manufacturer also needs to prepare different types of excessive heating protection circuit depending on the model. This means, the manufacturer needs to prepare many types of substrate for the excessive heating protection circuits, and inventory control for all these substrates is extremely costly to the manufacturer.